Among the various fungal strains screened for citric acid production, Aspergillus niger is known to produce considerable amounts of citric acid and other organic acids when cultivated in carbohydrate-rich medium in solid substrate fermentation (SSF). Since A. niger on a solid substrate grows under conditions similar to the natural habitat, SSF is ideal to cultivate A. niger for the purpose of producing citric acid. / An initial optimization (study 1) was conducted in batch type fermentation experiments using peat moss supplemented with glucose to simulate an organic waste. The effects of various nutrients (glucose, (NH4)2SO 4, KH2PO4 and NaCl), fermentation parameters (moisture content, temperature, inoculum density, composition of solid substrate and particle size) and of initial level of potential stimulators (ethanol, methanol, phytate and surfactant) were evaluated with respect to citric acid production by A. niger grown on damp peat moss. In these experiments, optimization using a traditional 'one-factor-at-a-time' method was applied to determine key factor ranges for the production of citric acid. When the fermentation was carried out using the final optimal conditions. This allowed for a 50-fold increase in citric acid production compared to the production of citric acid by A. niger grown on peat moss supplemented with 100 g glucose/kg DPM. / A second set of experiments (study 2) was conducted to optimize fermentation conditions for citric acid production in a column bioreactor. A. niger NRRL 567 grown on damp peat moss was held within a column bioreactor and periodically irrigated with a glucose-rich solution simulating field conditions. Three variables including aeration, thickness of solid substrate bed and incubation temperature were optimized using a 23 full factorial design (FFD). Under optimum, the total citric acid production and yield were 120.6 g/kg DPM and 18.5% respectively. / A third experiment (study 3) compared the production of citric acid by A. niger in submerged fermentation using cheese whey, as opposed to batch and semi-continuous fermentation using peat moss. Various fermentation conditions such as nutrients (glucose, (NH4)2SO 4 and KH2PO4), stimulators (methanol, olive oil and phytate) and fermentation parameters (pH, fermentation time and inoculum density) were optimized using a central composite design (CCD). Citric acid production improved citric acid production by a factor of 13.3 when compared to the production of citric acid by A. niger NRRL 567 using whey-based medium (50 g/l) alone. / As compared to submerged and semi-continuous fermentation, Batch type SSF could take higher levels of initial glucose and produce the high concentration of citric acid within a shorter period of time. Thus, SSF may be considered to be better technique than submerged fermentation, if main disadvantage like non-homogeneous fermentation conditions could be overcome.

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Thesis (M.Sc. (Animal Production)) -- University of Limpopo, 2023 / The development curve of Venda chicken fed with various amounts of citric acid was evaluated using nonlinear models in an experiment. to ascertain the effects of citric acid supplementation level on feed intake, body weight increase, and linear measurements on Venda chickens. 200 male Venda chickens were used in the experiment which lasted 90 days. The chicks were randomly assigned to four treatments (0, 12.5, 25 and 50g of citric acid inclusion) with 5 replications, resulting in 20 floor pens with 10 chicks per replicate. A completely randomized design was employed as experimental design. Analysis of Variance (ANOVA) was used to determine the effect of citric acid on feed intake, body weight gain, FCR, GR and body linear measurements. Three different non-linear models, namely Gompertz, Weibull, and Richards, were used to define the growth curves of the Venda chickens. The Duncan multiple range test at the 5% level of significance was utilized to detect significant differences between the means. Models were compared using coefficients of determination (R2) and standard errors (SE). The results indicated that feed intake, body weight, average daily gain and growth rate of Venda chickens were not affected (p > 0.05) by citric acid supplementation levels. Similarly, Citric acid supplementation had no effects (p > 0.05) on the shank length and wing length of Venda chickens. However, feed conversion ratio (FCR) of Venda chickens was improved (p < 0.05) by with an increase in citric acid supplementation. The Venda chicken fed citric acid at grower phase and finisher phase had better growth performances than the starter phase. Citric acid supplementation improved the back length and thigh length of Venda chickens. The Venda chickens fed citric acid at 25g inclusion level significantly higher back and thigh length. The coefficient of determination ranged from 0.00 to 0.98 in all the treatments. The Gompertz Model and Richards Models both exhibited the same coefficient of determination across all treatments. The model with the lowest standard error was found to best describe the growth curve of male Venda across all treatments. The Gompertz model was observed to be suitable for explaining the growth of Venda chickens fed with feed without citric acid (CA0g) and citric acid 125g inclusion (CA12.5g). The Richards model was observed to be suitable for explaining the growth of Venda chickens fed citric acid 25g inclusion (CA25g) and citric acid 50g inclusion (CA50g). It is recommended to use a supplement containing lower citric acid to enhance the body linear measurements and growth performance of the chickens. Gompertz and Richards models can be utilized to characterize the growth curve of Venda chickens. / National Research Foundation (NRF)